U.S. patent number 4,832,459 [Application Number 07/060,931] was granted by the patent office on 1989-05-23 for backlighting for electro-optical passive displays and transflective layer useful therewith.
This patent grant is currently assigned to Rogers Corporation. Invention is credited to William P. Harper, Michael S. Lunt.
United States Patent |
4,832,459 |
Harper , et al. |
May 23, 1989 |
**Please see images for:
( Certificate of Correction ) ** |
Backlighting for electro-optical passive displays and transflective
layer useful therewith
Abstract
In an electroluminescent lamp having a phosphor layer disposed
between corresponding lamp electrodes adapted to apply an
excitation potential to cause the phosphor layer to emit light, a
front lamp electrode which, in addition to being light transmissive
to radiation from the phosphor layer, consists of a thin layer of
light-transmissive binder containing a distribution of discrete
particles that are characteristically light-reflective and
electrically conductive. The electrode particles provide aggregate
diffuse outward reflectance sufficient to serve as a reflector
behind an LCD display for ambient light that falls upon the LCD
display to illuminate the display under light ambient conditions.
Furthermore, the particles in the electrode layer are adapted to
contribute to electrical continuity through the layer sufficient to
apply excitation to the phosphor layer, and the electrode layer
particles have sufficient spaces between them to provide
escape-paths for light from the excited phosphor layer to
back-illuminate the LCD display under dark ambient conditions. A
method of forming the improved electrode, e.g. by shear transfer
techniques, is also described. Certain aspects of the invention
apply to the function of improved transflective layers, per se.
Inventors: |
Harper; William P. (South
Killingly, CT), Lunt; Michael S. (Abbington, CT) |
Assignee: |
Rogers Corporation (Rogers,
CT)
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Family
ID: |
27369940 |
Appl.
No.: |
07/060,931 |
Filed: |
May 27, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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633123 |
Jul 20, 1984 |
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577145 |
Feb 6, 1984 |
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Current U.S.
Class: |
349/68; 313/503;
349/114; 349/69 |
Current CPC
Class: |
G02F
1/133617 (20130101); H05B 33/10 (20130101); H05B
33/12 (20130101); H05B 33/20 (20130101); H05B
33/28 (20130101) |
Current International
Class: |
G02F
1/1335 (20060101); G02F 1/13 (20060101); H05B
33/26 (20060101); H05B 33/10 (20060101); H05B
33/12 (20060101); H05B 33/20 (20060101); H05B
33/28 (20060101); G02F 001/13 () |
Field of
Search: |
;350/336,338,345
;313/503,504,505,510,511 ;250/483.1,486.1,487.1 ;252/503 ;428/917
;427/66 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1059678 |
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Jul 1979 |
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CA |
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2355134 |
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May 1975 |
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DE |
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2449602 |
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Apr 1976 |
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DE |
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2722388 |
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Mar 1979 |
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DE |
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2808260 |
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Aug 1979 |
|
DE |
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2856170 |
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Jun 1980 |
|
DE |
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0828720 |
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1960 |
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GB |
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Primary Examiner: Miller; Stanley D.
Assistant Examiner: Thantu; Napoleon
Parent Case Text
This is a continuation of application U.S. Ser. No. 633,123, filed
July 20, 1984, now abandoned, which is a continuation-in-part of
application U.S. Ser. No. 577,145, filed Feb. 6, 1984.
Claims
What is claimed is:
1. An electro-optical display device comprising an
electroluminescent lamp having a phosphor layer disposed between
corresponding lamp electrodes that are adapted to apply an
excitation potential to cause said phosphor layer to emit light,
the front lamp electrode being light-transmissive to radiation from
said phosphor layer, and,
positioned in front of said front electrode, a passive liquid
crystal display device having a display defined by an array of a
large number of discrete picture elements activatable selectively
to form desired characters,
said front lamp electrode comprising a thin layer of a
light-transmissive, non-crosslinked, polyvinylidene fluoride binder
containing a uniform distribution of discrete metal flakes in
sufficient electrical contact with each other to at least assist in
applying said excitation potential to said phosphor layer,
said flakes characterized as being spaced apart from each other and
having diameters between 5 and 10 .mu.m in the direction of the
plane of the layer in which they reside and surfaces that are not
uniformly flat or smooth, all so as to create a large multiplicity
of tiny, diffusely reflective surfaces that reflect a sufficient
amount of light to illuminate said liquid crystal display under
light ambient conditions and a large multiplicity of open passages
between said reflective surfaces that permit light to escape from
said phosphor layer through said front lamp electrode in an amount
sufficient to back-illuminate said liquid crystal display under
dark ambient conditions.
2. The electro-optical device of claim 1 characterized as
providing, for all ambient conditions, a display contrast ratio of
at least about 2:1.
3. In an electroluminescent lamp comprising a phosphor layer
disposed between corresponding lamp electrodes that are adapted to
apply an excitation potential to cause said phosphor layer to emit
light, the front lamp electrode being light-transmissive to
radiation from said phosphor layer,
the improvement wherein
said front lamp electrode comprises a thin layer of a
light-transmissive, non-crosslinked, polyvinylidene fluoride binder
containing a uniform distribution of discrete metal flakes in
sufficient electrical contact with each other to at least assist in
applying said excitation potential to said phosphor layer,
said flakes characterized as being spaced apart from each other and
having diameters between 5 and 10 .mu.m in the direction of the
plane of the layer in which they reside and having surfaces that
are not uniformly flat or smooth, all so as to create a large
multiplicity of tiny, diffusely reflective surfaces that reflect a
sufficient amount of light to illuminate a liquid crystal display
under light ambient conditions and a large multiplicity of open
passages between said reflective surfaces that permit light to
escape from said phosphor layer through said front lamp electrode
in an amount sufficient to back-illuminate a liquid crystal display
under dark ambient conditions.
4. The electroluminescent lamp of claim 3 wherein the amount of
light reflected from said front lamp electrode is about 80 percent
of the ambient light incident upon the front of said electrode.
5. The electroluminescent lamp of claim 3 wherein the light emitted
through said front lamp electrode has a brightness of at least one
foot-lambert.
Description
BACKGROUND OF THE INVENTION
Passive liquid crystal display devices are made of a layer of
liquid crystal disposed between opposed electrodes activatable in
segments. Light passing into the device through the front
electrode, which is transparent, is internally reflected from the
back of the device, e.g. from a reflective film or coating. As is
well known, the light entering at activated segments of the device
is modified by the liquid crystals to provide a contrasting visual
effect relative to other areas. To obtain a practical display that
corresponds to the shape of the activated regions, a contrast ratio
of about 2:1 is required where the display and background are of
the same color. The contrast ratio varies proportionally with
ambient light, assuming a constant electrode potential. For this
reason, where liquid crystal devices are desired to be used at
night or at low ambient light levels, efforts have been made to
provide additional lighting.
Those suggestions which employ an incandescent light source, or
other point light source, must contend with the problems of
nonuniformity of illumination. Examples of Castleberry U.S. Pat.
No. 4,212,048, which employs a transmissive diffuser; Brooks U.S.
Pat. No. 4,206,501 and Eberhardt U.S. Pat. No. 4,229,783, which
employ diffusers for bulbs located behind the display, Eberhardt
having an additional reflective front surface to facilitate
diffusion of the ambient light; and Borden, Jr., U.S. Pat. No.
3,748,018, which employs a reflector to direct light from the
source onto the rear surface of the display.
Others have sought to employ electroluminescent lamps behind the
display devices, with selective reflectivity provided in the
ambient light mode by means of a dichroic wavelength selective
reflector (Aldrich U.S. Pat. No. 3,869,195), or a microlouvered
light control film (Myer U.S. Pat. No. 3,811,751) or by utilizing
the reflective rear electrode of the EL lamp itself (Saurer et al.
U.S. Pat. No. 4,138,195). These have various drawbacks such as
substantial expense and lack of sufficient contrast in one or the
other of the modes of operation.
It is an objective of this invention to provide a simple, low cost
and effective device for uniformly illuminating, from the back, a
passive liquid crystal display for viewing at night or under
conditions of low ambient light by illumination from the rear; it
is also an objective to provide a device useful with a passive
liquid crystal display, to allow the display to be seen in all
levels of light and darkness; a further objective is to provide an
improved transflector and method for its fabrication.
SUMMARY OF THE INVENTION
Certain aspects of the invention relate to an electroluminescent
lamp comprising a phosphor layer disposed between corresponding
lamp electrodes that are adapted to apply an excitation potential
to cause the phosphor layer to emit light, the front lamp electrode
being light-transmissive to radiation from the phosphor layer.
According to these aspects of the invention, the front lamp
electrode comprises a thin layer of light-transmissive binder
containing a distribution of discrete particles that are
characteristically light-reflective and electrically conductive,
the particles providing aggregate diffuse outward reflectance
sufficient to serve as a reflector behind an LCD display for
ambient light that falls upon the LCD display to illuminate the
display under light ambient conditions, the particles in the layer
being in sufficient electrical continuity, preferably as a result
of particle-to-particle contact, to apply excitation to the
phosphor layer, and the particles having sufficient spaces between
them to provide outward escape-paths for light from the excited
phosphor layer to back-illuminate the LCD display under dark
ambient conditions.
In preferred embodiments, the particles in the front lamp electrode
have diameters of the order of about 10 microns in the direction of
the plane of the layer in which they reside; the particles in the
front lamp electrode have the form of metal flakes, the particles
being generally aligned, on average, with the plane of the layer in
which they reside; the binder is a polymer consisting essentially
of polyvinylidene fluoride (PVDF); the particle distribution in the
front lamp electrode provides a reflectance of about 80 percent to
ambient light incident upon the front of the electrode; and the
lamp is adapted to emit radiation through the front lamp electrode
of brightness of at least one foot-lambert.
According to another aspect of the invention, an electro-optical
device is comprised of the combination of the electroluminescent
lamp as described above with a display of an array of a large
number of discrete LCD picture elements activatable selectively to
form desired characters, the front lamp electrode being arranged to
serve as a rear reflective surface for the LCD display under light
ambient conditions, and the lamp being adapted to provide light
through the front lamp electrode to back-illuminate the display
under dark ambient conditions.
In preferred embodiments of this aspect of the invention, the
average surface area of a particle in the front lamp electrode is
less than about one percent of the surface area of a picture
element, and the device is capable of providing, for all ambient
conditions, a display contrast ratio of at least about 2:1.
Other aspects of the invention involve methods of forming the front
electrode described above. In one aspect, the method comprises
depositing upon the phosphor layer at least one thin layer of a
suspension of light-transmissive polymer solid dispersed in a
liquid phase containing a uniform dispersion of discrete particles
that are characteristically light-reflective and electrically
conductive, and causing the layer to fuse throughout to form a
continuous electrode layer, the discrete particles of the electrode
layer being uniformly distributed throughout and providing
aggregate diffuse reflectance sufficient to serve as a reflector
behind an LCD display for ambient light to illuminate the LCD
display under light ambient conditions, these particles making
sufficient contact with each other in their layer to provide
electrical continuity sufficient to apply excitation to the
phosphor layer, and the particles having sufficient spaces between
them to enable escape of light from the excited phosphor layer
through the front lamp electrode to back-illuminate the LCD display
under dark ambient conditions.
In another aspect, the method of forming a transflective layer
comprises depositing by shear transfer and drying at least one thin
layer of a suspension of light transmissive polymer solid dispersed
in a liquid phase, the layer containing a uniform dispersion of
discrete particles that are characteristically light-reflective,
and for use, e.g. as an electrode, electrically conductive, and the
method including heating to fuse the polymer particles continuously
throughout the extent of the layer. Preferably, the layer is
deposited by shear transfer upon a fused first layer comprising the
polymer solid, and the heating causes the layers to fuse
continuously throughout and between the layers to form a monolithic
unit; and the layer is deposited by silk screen printing or doctor
blade coating.
In preferred embodiments of both methods, the discrete particles
have the form of flakes and during the drying of the layer a large
multiplicity of the particles are caused to generally align
themselves with the extent of the plane of the thin layer; and the
polymeric binder consists essentially of polyvinylidene fluoride
(PVDF).
According to another aspect of the invention, a transflector
comprises a thin layer of light-transmissive polymer binder
consisting essentially of polyvinylidene fluoride (PVDF) containing
a distribution of discrete particles that are characteristically
light reflective, the particles in the layer providing aggregate
diffuse reflectance.
Preferably, at least some of the particles are electrically
conductive and serve to conduct electricity through the layer. In
preferred embodiments, the transflector is provided in combination
with an electroluminescent lamp, disposed over the phosphor layer
of the lamp, and the layer serves at least in part as the front
electrode of the lamp.
PREFERRED EMBODIMENT
We first briefly describe the drawings:
FIGS. 1 and 2 are diagrammatic perspective views of an
electro-optical device according to the invention in light and dark
ambient conditions, respectively;
FIGS. 1a and 2a are diagrammatic edge views of the device of FIGS.
1 and 2;
FIGS. 3 and 3a are plan and side section representations,
respectively, of a magnified view of a portion of the front lamp
electrode; and
FIG. 4 is a diagrammatic representation of an electro-optical
device of the invention showing light paths in light and dark
ambient conditions.
STRUCTURE AND OPERATION
Referring to FIGS. 1 and 2, electro-optical device 10 comprises a
typical liquid crystal display (LCD) 12 of the twisted nematic
type, e.g. as described in Klein U.S. Pat. No. 3,612,654, and a
correspondingly sized and shaped electroluminescent lamp 14. The
lamp 14 and LCD 12 are held in a fixed relationship by frames 11,
shown in dashed line in FIGS. 1a and 2a.
In FIG. 1, source 16 of ambient light is radiating light toward the
front surface 18 of the LCD. Referring to FIG. 1a, ambient light
rays, L.sub.A, pass through, in sequence; anti-reflective coating
20, transparent front substrate 22, anti-reflective coating 24,
transparent front display electrode 26, liquid crystal 28 (lying
between the front electrode and passivating layer 30 in a
compartment sealed about the periphery by gasket 32), transparent
rear display electrode 34 and transparent rear substrate 36. The
light rays passing out of the rear face 38 of the LCD cross gap 40,
of about 0.005 inch, between display 12 and electroluminescent lamp
14, which is not activated, to enter the lamp via front surface 42,
passing through lamp transparent protective layer 44 and into front
electrode 46.
Referring to FIGS. 2, 2a, 3 and 3a, the front lamp electrode 46 is
a layer comprised of a light-transmissive plastic binder 48 of,
e.g., polyvinylidene fluoride (PVDF), and silver flakes 50
dispersed evenly throughout, the planes of the flakes being
generally in alignment with the plane of extent of the electrode,
see FIG. 3a, and presenting an aggregate frontal area of about 80
percent of the electrode area, with light-transmissive spaces 51
being defined between some of the flakes. The flakes are formed,
e.g. by passing pellets through a ball or hammer milling process,
thus preferably they are not uniformly smooth or flat, but have
somewhat irregular, and bent surfaces similar to that of, e.g.,
cornflakes. The aggregate effect of such metal flakes in the layer
is to provide diffuse reflectance, which is important to enable
viewing of the LCD over the normal angular range.
For ease of illustration, in FIG. 4 only certain components of the
electro-optical device are shown: for LCD 12, the front and rear
transparent electrodes 26, 34, the liquid crystal layer 28, and the
rear reflector layer 46, which is also the front electrode for lamp
14; for lamp 14, the phosphor-containing layer 60 disposed between
front and rear lamp electrodes 46, 112.
Referring to FIGS. 1a and 4, the ambient light rays, L.sub.A, that
have passed through the LCD display penetrate through the binder of
the front lamp electrode 46 until reflected by the surfaces of the
silver flakes. While being generally aligned with the plane of the
layer, the flakes are somewhat randomly distributed. They generally
are of sufficient thickness to be opaque to visible light, i.e.
they have thickness five (preferably substantially more) times the
wavelength of visible light. The major portion of the ambient light
rays, L.sub.A, entering the front lamp electrode 46, e.g. about 80
percent, are reflected out of the electrode as reflected light
rays, L.sub.R. The semi-random distribution of the flakes and their
irregular surfaces result in a general diffuse reflection of light
over a range of angles that is suitable for LCD backlighting. Due
to the open spaces 51, FIG. 3, between flakes, a minor portion,
L.sub.P, of the light rays passes through the electrode layer
between flakes and is lost. A second minor portion, L.sub.D, of the
light rays that is reflected from the front surfaces of the silver
flakes is re-reflected by the back surfaces of other flakes lying
thereabove in the electrode layer, and also passes through spaces
51 of the electrode and is lost.
The reflected light rays, L.sub.R, travel a reverse course to that
of entering light, out of lamp electrode 46 and through the LCD 12.
Light rays L.sub.R and L.sub.A that are incident upon activated
sections 29 of the liquid crystals (activated through the action of
corresponding segments 26', 34' of the front and rear display
electrodes 26, 34, powered by connecting wires 60, 62 from
controlled source 64), are relatively modified in a manner
well-known in LCD technology to generate to the observer 52 a
contrasting light and dark display.
A large multiplicity of discrete LCD picture elements ("pixels")
activatable according to a predetermined pattern form desired
characters. For example, in FIGS. 1 and 2, 5 by 7 pixel grids are
shown, with 12 pixels in the grid activated to display the numeral
"4". Each pixel is about 300 microns in diameter. As the silver
flake particles 50 have average diameter of between about 5 and 10
microns (compared to thickness between about 0.25 and 2.0 microns)
and surface areas well less than about one percent of the surface
area of each pixel, the electrode of the corresponding portion of
the EL lamp includes a very large multiplicity of tiny reflective
surfaces and open passages. Thus the resulting display is created
by the combined reflectance (or emittance) of a large number of
elements, to achieve desired resolution. The results are further
improved by the diffuseness of the reflectance from the particles,
the diameter of the flakes, in fact, being much less than the
discrete surfaces of certain diffusers that heretofore have been
employed with LDCs.
In FIG. 2, the electro-optical device 10 is shown in dark ambient
conditions, with no light rays directed toward the front surface 18
of the LCD 12. Referring also to FIGS. 2a and 4, the
electroluminescent lamp 14 has been activated by applying an
electrical potential (from source 138 via connecting wires 134,
136) between front lamp electrode 46, described above, and rear
lamp electrode 112, across the electroluminescent layer 60 of
phosphor particles 56 uniformly dispersed in PVDF binder 58. (The
electroluminescent layer is separated from the rear electrode 112
by an insulator layer 54 (FIG. 2a) comprises of barium titanate
particles dispersed in PVDF binder.) Referring to FIGS. 3 and 3a,
the silver flake particles are present in a quantity to make
sufficient edge-to-edge contact in the layer to provide adequate
electrical continuity to apply electrical excitation to the
phosphor layer. Thus excited, the phosphor particles emit light
rays, L.sub.E, a portion of which, having brightness between about
5 to 100 ft-lamberts, enter the front lamp electrode 46. Referring
again to FIG. 4, many of the light rays, L.sub.IR, again about 80
percent, entering the electrode are internally reflected back into
the phosphor layer; however, a sufficient portion of the light
rays, L.sub.O, having brightness of about 1 to 5 ft-lamberts,
passes out of the electrode into the protective layer. Rays L.sub.O
pass across gap 40, enter LCD 12 via rear surface 38, and then
proceed through rear display substrate 36, transparent rear
electrode 34, passivating layer 30, liquid crystals 28 (being
affected by the activated segments), transparent front display
electrode 26 and transparent front substrate 22 with its
anti-reflective coatings 20, 24. Observer 52' (FIG. 2) in darkness
again observes a contrasting light and dark display.
CONTRAST EVALUATION
To be noticeable to an observer, the light and dark areas of the
display must have a minimum level of contrast, e.g. a contrast
ratio of 2:1 typically is sufficient.
To establish the efficiency of the device described above, the
contrast ratio of light to dark areas of display was evaluated
under different ambient light conditions and compared to the
performance of the same model LCD with a standard metallized foil
reflector on the rear surface.
Measurements were conducted using a Pritchard Photometer, Model
1980A-OP, with the photo-optic setting open and the focus spot at 6
degrees, under four conditions of ambient light: darkened room with
black walls (photo darkroom), closed room with the lights off,
closed room with ceiling lights on, and under bright direct
lighting (desk lamp). The level of ambient light present during
each of the four tests was determined, then the photometer test
spot was focused sequentially on the dark display area and on the
light reflective background and the contrast ratio calculated. The
results of the evaluation are summarized in Table A. In all tests,
the display was powered at 60 volts, 400 hertz. In Tests 1 and 2,
the lamp was powered at 115 volts, 1000 hertz; in Tests 3 and 4,
the lamp was turned off.
TABLE A ______________________________________ Standard LCD LCD/EL
Device(10) ______________________________________ Test 1 - Complete
Darkness Background Light 0 ft-Lamberts Character Area (Dark) 0
0.21 Backplane (Light) 0 0.59 Contrast Ratio 0 2.8:1 Test 2 -
Semi-Darkness Background Light 0.90 ft-Lamberts Character Area
(Dark) 0.20 0.25 Backplane (Light) 0.42 0.75 Contrast Ratio 2.1:1
3.0:1 Test 3 - Low Light Background Light 32.0 ft-Lamberts
Character Area (Dark) 1.48 0.83 Backplane (Light) 4.65 3.10
Contrast Ratio 3.1:1 3.7:1 Test 4 - Bright Light Background Light
150-200 ft-Lamberts Character Area (Dark) 21.0 9.8 Backplane
(Light) 82.0 47.0 Contrast Ratio 3.9:1 4.8:1
______________________________________
As shown, the display on device 10 of the invention was observable
at all external light levels, while the display on the standard LCD
has borderline contrast in semi-darkness, i.e. dark ambient
conditions, and could not be seen when there was no ambient
light.
LAMP MANUFACTURE
Referring to FIG. 2a, we now describe an electroluminescent lamp 14
formed of a superposed series of the layers including the novel
reflective-transmissive conductive front electrode 46 of the
invention.
The substrate 112 used in this lamp configuration was copper
(0.0014 inch thick, one ounce) cut to size and shape corresponding
to that of the LCD device 12 with which the lamp was to be used,
e.g. 3 inches by 4 inches.
DIELECTRIC INSULATING LAYER (54)
A coating composition for forming dielectric layer 54 upon the
substrate 112, in this case to act as an insulator between the
substrate/electrode 112 and the overlying light-emitting phosphor
layer 60 (described below), was prepared as follows:
To prepare the dielectric composition, 10 grams of a PVDF
dispersion of 45 percent, by weight, polyvinylidene fluoride (PVDF)
in a liquid phase believed to be primarily carbitol acetate
(diethyl glycol monoethyl ether) were measured out. This dispersion
was obtained commercially from Pennwalt Corporation under the
tradename "Kynar Type 202". As the electrical property-imparting
additive, 18.2 grams of barium titanate particles (BT206 supplied
by Fuji Titanium, having a particle size of less than about 5
microns) were mixed into the PVDF dispersion. An additional amount
of carbitol acetate (4.65 grams) was added to the composition to
maintain the level of solids and the viscosity of the composition
at a proper level to maintain uniform dispersion of the additive
particles while preserving the desired transfer performance. It was
observed after mixing that the composition was thick and creamy and
that the additive particles remained generally uniformly suspended
in the dispersion without significant settling during the time
required to prepare the example. This is due, at least in part, to
the number of solid PVDF particles (typically about 2 microns in
diameter) present in the composition.
The composition was poured onto a 320 mesh polyester screen
positioned 0.145 inch above the substrate, selected for its
resistance to the carrier fluid employed and for its ability to
withstand the extreme temperatures of treatment, e.g. up to
500.degree. F., as described below. Due to its high apparent
viscosity, the composition remained on the screen without leaking
through until the squeegee was passed over the screen, exerting
shear stress on the fluid composition causing it to shear-thin due
to its thixotropic character and pass through the screen to be
printed, forming a thin layer on the substrate below. The deposited
layer was subjected to drying for 21/2 minutes at 175.degree. F. to
drive off a portion of the liquid phase, and was then subjected to
heating to 500.degree. F. (above the initial melting point of the
PVDF) and was maintained at that temperature for 45 seconds. This
heating drove off remaining liquid phase and also fused the PVDF
into a continuous smooth film on the substrate.
The resulting thickness of the dried polymeric layer was 0.35 mil
(3.5.times.10.sup.-4 inch).
A second layer of the composition was screen-printed over the first
layer on the substrate. The substrate now coated with both layers
was again subjected to heating as above. This second heating step
caused the separately applied PVDF layers to fuse together. The
final product was a monolithic dielectric unit having a thickness
of 0.7 mil with no apparent interface between the layers of
polymer, as determined by examination of a cross-section under
microscope. The particles of the additive were found to be
uniformly distributed throughout the deposit.
The monolithic unit 54 was determined to have a dielectric constant
of about 30.
LIGHT EMITTING PHOSPHOR LAYER (60)
A coating composition for forming the light emitting phosphor layer
60 was prepared as follows:
To prepare the composition, 18.2 grams of a phosphor additive, zinc
sulfide crystals (type #723 from GTE Sylvania, smoothly rounded
crystals having particle size of about 15 to 35 microns) were
introduced to 10 grams of the PVDF dispersion used above. It was
again observed after mixing that despite the smooth shape and
relatively high density of the phosphor crystals, the additive
particles remained uniformly suspended in the dispersion during the
remainder of the process without significant settling.
The composition was superposed by screen printing over the
underlying insulator layer 54 through a 280 mesh polyester screen
positioned 0.145 inch above the substrate to form a thin layer. The
deposited layer was subjected to the two stage drying and fusing
procedure described above. Subjecting the layers to temperatures
above the melting temperature of the PVDF material caused the PVDF
to fuse throughout the newly applied layer and between the layers
to form a monolithic unit upon the substrate 112. However, the
interpenetration of the material of the adjacent layers having
different electrical properties was limited by the process
conditions to less than about 5 percent of the thickness of the
thicker of the adjacent layers, i.e. to less than about 0.06 mil so
that the different electrical property-imparting additive particles
remained stratified within the monolithic unit as well as remaining
uniformly distributed throughout their respective layers.
The resulting thickness of the dried polymeric layer was 1.2 mils
(1.2.times.10.sup.-3 inch).
The deposited film was tested and found to be uniformly
photoluminescent, without significant light or dark spots.
TRANSFLECTIVE/CONDUCTIVE FRONT LAMP ELECTRODE (46)
The coating composition for forming the novel
transflective/conductive front lamp electrode 46 of the invention
was prepared as follows:
To prepare this conductive composition, 13.52 gram of silver flake
(Metz silver Flake #7 from Metz Metallurgical Corporation, So.
Plainfield, NJ, having Scott apparent density of 27.2 gm/in.sup.3
and TAP density by Tap-Pak of 3.1 gm/cc) were added to 55.80 grams
of the PVDF dispersion used above. It was again observed after
mixing that the additive particles remained uniformly suspended in
the dispersion during the remainder of the process without
significant settling.
The composition was superposed by the screen printing through a 320
mesh polyester screen positioned 0.15 inch above the light-emitting
phosphor layer 60. The substrate with the multiple layers coated
thereupon was again heated to above the PVDF melting temperature to
cause the transflective/conductive front electrode layer to fuse
throughout to form a continuous uniform layer and to fuse this
layer together with the underlying light-emitting layer to form a
monolithic unit. The front electrode layer thus formed is adapted
to serve as both a conductor and as a reflector, and as the flakes
are uniformly dispersed throughout the binder, a level of light
transmissivity is achieved through the deposited layer.
The resulting thickness of the dried polymeric layer was 0.5 mil
(0.5.times.10.sup.-3 inch).
The deposited layer was tested and found to have conductivity of
125 ohm-cm, and have light transmissivity of about 1-5 ft-Lamberts,
about 80% of the transmissivity anticipated of an EL lamp of
similar construction with a light transmissive,
indium-oxide-containing, transparent front electrode.
CONDUCTIVE BUSS
The coating composition for forming a conductive buss 120 to
distribute current via relatively short paths to the electrode was
prepared as follows:
To prepare this conductive composition, 15.76 grams of silver flake
(from Metz Metallurgical Corporation, of 325 mesh #7 particle size)
were added to 10 grams of the PVDF dispersion used above. The
particles remained uniformly suspended in the dispersion during the
remainder of the process without significant settling.
The composition was screen printed through a 320 mesh polyester
screen positioned 0.15 inch above semi-transparent upper electrode
46 as a thin narrow bar extending along one edge of the electrode
layer. The deposited layer was subjected to the two stage drying
and fusing procedure described above to fuse the PVDF into a
continuous smooth film with the silver flake uniformly distributed
throughout.
The resulting thickness of the dried polymeric layer was 0.3 mil
(0.3.times.10.sup.-3 inch).
The deposited film was tested and found to have conductivity of
10.sup.-3 ohm-cm.
This construction with connecting wires 134, 136 (FIG. 2a) and a
power source 138, forms a functional electroluminescent lamp 14.
Electricity is applied to the lamp via the wires and is distributed
by the buss layer to the upper electrode 46 to excite the phosphor
crystals in the underlying layer 60, which causes them to emit
light.
Due, however, to the damaging effect of moisture on phosphor layer
60, it is desirable to add a moisture impervious protective and
insulative layer 44 about the exposed surfaces of the layers of the
lamp to seal to the peripheral surface of the substrate 112. This
layer 44 is also formed according to the invention, as follows:
PROTECTIVE-INSULATIVE LAYER (44)
The light transmissive, i.e., clear, PVDF dispersion employed
above, devoid of electrical-property additives, is screen printed
over the exposed surfaces of the lamp 14 through a 180 mesh
polyester screen. The lamp was dried for two minutes at 175.degree.
F. and heated for 45 seconds at 500.degree. F. The coating and
heating procedure was performed twice to provide a total dried film
thickness of protective-insulative layer 44 of 1.0 mils. (By using
PVDF as the binder material in this and all the underlying layers,
each layer has the same processing requirements and restrictions.
Thus the upper layers, and the protective coating, may be fully
treated without damage to underlying layers, as might be the case
if other different binder systems were employed.)
The final heating step results in an electroluminescent lamp 14 of
cross-section as shown in the figures. The polymeric material that
was superposed in layers upon flexible substrate 112 has fused
within the layers and between the layers to form a monolithic unit
about 3.4 mils thick that flexes with the substrate. As all the
layers are formed of the same polymeric material, all the layers of
the monolithic unit have common thermal expansion characteristics,
hence temperature changes during testing did not cause
delamination. Also, due to the continuous film-like nature of each
layer due to the fusing of its constituent particles of PVDF and
the interpenetration of the polymeric material in adjacent layers,
including the protective layer 44 covering the top and exposed side
surfaces, the lamp is highly resistant to moisture during high
humidity testing, and the phosphor crystals did not appear to
deteriorate prematurely, as would occur if moisture had penetrated
to the crystals in the phosphor layer.
OTHER EMBODIMENTS
Numerous other embodiments are within the following claims, as will
be obvious to one skilled in the art.
The electroluminescent lamp may be selectively powered
independently of the LCD, or may be powered via the same switch so
the lamp is activated at all times while the LCD is on. In this
state, the light rays emitted by the phosphor layer 60 and passing
through the front lamp electrode 46 will supplement the reflected
light rays, L.sub.R, and, at least in part, replace light ray,
L.sub.D and L.sub.P, lost through the electrode.
Other materials have sufficient conductivity and reflectivity for
use according to the invention. These include, e.g., copper and
bronze. Silver coated beads might also be employed. Where it is
desirable to increase the conductivity of the electrode without
significantly affecting the reflectivity, semi-transparent
conductive particles, e.g. indium oxide, may be added to the binder
material. Indium oxide or other semi-transparent conductive
material might also be used where the level of silver flake present
does not provide adequate physical flake-to-flake contact for
electrical continuity through the electrode layer, the indium oxide
particles acting to conduct current between the flakes.
A further reflective-transmissive-conductive layer might also be
formed, e.g. over the flake-containing layer, as a thin layer of
metal, e.g., applied by sputtering or evaporation to a front
surface of the EL lamp.
A transflective layer having limited or no electrical conductivity
may also be formed according to the invention, e.g. for use with
another front lamp electrode to improve light transmission and
electrical conductivity.
The protective layer 44 of the electroluminescent lamp may be
applied as preformed film of PVDF under pressure of 125 pounds per
square inch, and the lamp heated at 350.degree. F. for one minute
and then cooled while still under pressure. Each separate layer
applied may have a dry thickness of as much as 0.010 inch, although
thickness in the range between about 0.003 inch to 0.0001 inch is
typically preferred. Similarly the conductive layer 46 may be
formed on the pre-formed protective layer.
As mentioned, the composition for the transflective layer 46 as
well as for other layers may be applied by screen printing, or by
various of the doctor blade coating techniques, e.g. knife over
roll or knife over table, or by other means. The shear-imparting
conditions of screen printing may also be varied, e.g. the squeegee
may be advanced along the screen at rates between about 2 and 200
inches per minute, and the size of the screen orifices may range
between about 1.4 and 7 mils on a side.
While materials which consist essentially of homopolymers of PVDF
are preferred, other materials may be employed. The guiding
criteria for selection are low moisture absorptivity, ability of
particles to fuse at elevated temperature to form a continuous
moisture barrier film, and, when applied to flexible substrate,
flexibility and strength.
The liquid phase of the composition may be selected from the group
of materials categorized in the literature as "latent solvents" for
PVDF, i.e., those with enough affinity for PVDF to solvate the
polymer at elevated temperature, but in which at room temperature
PVDF is not substantially soluble, i.e., less than about 5 percent.
These include: methyl isobutyl ketone (MIBK), butyl acetate,
cyclohexanone, diacetone alcohol, diisobutyl ketone, butyrolactone,
tetraethyl urea, isophorone, triethyl phosphate, carbitol acetate,
propylene carbonate, and dimethyl phthalate.
Where additional solvation is desired, a limited amount of
"acetive" solvent which, in greater concentrations can dissolve
PVDF at room temperature, e.g., acetone, tetrahydrofuran (THF),
methyl ethyl ketone (MEK), dimethyl formamide (DMF), dimethyl
acetamide (DMAC), tetramethyl urea and trimethyl phosphate, may be
added to the carrier. Such a limited amount is believed to act
principally in the manner of a surfactant, serving to link between
the PVDF polymer particles and the predominant liquid phase, thus
to stabilize the PVDF powder dispersion.
As will also be obvious to a person skilled in the art, the
viscosity and weight percent of PVDF solids in the coating
composition may also be adjusted, e.g. to provide the desired
viscosity, suspendability and transfer characteristic to allow the
composition to be useful with additive particles of widely
different physical and electrical characteristics.
* * * * *